1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device provided with a group III-V nitride semiconductor layer and also with an end-face coating film formed on the end faces of a cavity.
2. Description of Related Art
In recent years, with the demands for increasingly high densities in the storage capacity of optical discs, there have been formulated standards for BD (Blu-ray Disc) and HD-DVD (high-definition DVD) employing blue semiconductor lasers, and there have been commercialized decoders and the like therefor. These newly developed discs offer higher densities (by being ready to be formed into two-layer discs) and allow fast writing. To benefit from these advantages, however, it is necessary to use high-reliability, high-output blue semiconductor lasers.
Conventionally, playback from and writing to CDs and DVDs is achieved by use of an AlGaAs-based or InGaAlP-based semiconductor laser, in which, to prevent degradation of the cavity end faces and to prevent optical damage to the cavity end faces, the cavity end faces are coated with a film of a dielectric such as SiO2, Si3N4, aluminum oxide, or the like. Disadvantageously, however, it has been confirmed that using this technology in blue semiconductor lasers results in a rapid increase in the drive current. This has been necessitating an improvement in the coating technology.
JP-A-2002-335053, asserting that one cause for degradation of the end faces is poor adhesion of the end face coating film, proposes forming the end face coating film on the cavity end faces with a metal adhesion layer laid in between.
Disadvantageously, however, using a metal film as the adhesion layer causes short-circuiting at the pn junction on the cavity end faces, and also leads to increased light absorption. Since a nitride semiconductor laser operates at a short lasing wavelength and the light it emits has high energy, even slight light absorption degrades the emission end face, making it impossible to realize a high-output device with a light output over 100 mW. On the other hand, from the viewpoints of avoiding short-circuiting at the pn junction and reducing light absorption, the film thickness there needs to be 10 nm or less, more preferably 5 nm or less, and particularly preferably 2 nm or less. This involves difficult control of the film thickness, resulting in diminished yields.
Another disadvantage is that, when an end face coating film formed of an oxide is formed on the cavity end faces directly or with a metal film formed as an adhesion layer in between, the oxygen contained in the end face coating film oxidizes the cavity end faces or the adhesion layer, and thereby lowers the lasing efficiency of the laser light. This not only increases the operating voltage and the power consumption, but also leads to lower durability.